The unique isozyme of lactate dehydrogenase, LDH-X, (LDH-C) is found in mature testes and spermatozoa of most mammals and some birds. The crystalline enzyme is both an auto-and alloantigen which impairs fertility by immunization of females. The objective of this research is to isolate antigenic peptides from tryptic digests of LDH-C4. The structure of these peptides will be determined in order to have them custom synthesized. Synthetic peptides will be conjugated to appropriated carrier molecules to render them immunogenic. Those peptides which provoke antibody formation to native LDH-C4 will be tested for anti-fertility properties.